One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO 90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent first electrode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode and the electroluminescent material. When a voltage is applied between the electrodes from a power supply one of the electrodes acts as a cathode and the other as an anode.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
For organic semiconductors, important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (HOMO) and the “lowest unoccupied molecular orbital” (LUMO) level. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in this field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve (peak) from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative.
The optical and electronic properties of an organic semiconductor are highly dependent on the energy of the aforementioned HOMO and LUMO levels. Furthermore, these energy levels are highly dependent on the chemical structure of the organic semiconductor. By selecting suitable materials, or combinations of materials, device performance can be improved.
For example, one way of improving efficiency of devices is to provide hole and electron transporting materials. WO 99/48610 discloses blending of hole transporting polymers, electron transporting polymers and electroluminescent polymers. A 1:1 copolymer of dioctylfluorene and triphenylamine is disclosed as a hole transporting polymer in this document. The type of charge transporting material which is most effective will be dependent on the HOMO and LUMO of the other components in the device.
Although there has been much improvement in the efficiency of devices using charge transporting materials, there is always a desire to develop new charge transporting materials to further improving efficiency when compared with existing devices.
WO 02/083760 discloses copolymers for use as charge transporting materials and fluorescent emissive materials in organic opto-electrical devices. The co-polymers comprise a first repeat unit which may be a triazine unit as shown in formula (a):

wherein R″ is selected from hydrogen, branched or linear C1-C20 alkyl or alkoxy.
The copolymers comprise a second repeat unit which may be selected from the group consisting of optionally substituted phenylenes, fluorenes, heteroaryls and triarylamines.
Triazines as components of blue electroluminescent devices are disclosed in WO 2004/077885.
U.S. Pat. No. 6,821,643 discloses arylated triazines that are deposited by evaporation to form a blue fluorescent light-emitting layer or an electron transport layer of an OLED.
U.S. Pat. No. 6,352,791 discloses arylated triazines that are deposited by evaporation to form an electron transport layer of an OLED.
WO 2005/105950 discloses certain tri-substituted triazines used as a blue fluorescent light-emitting layer of an OLED.
EP 1385221 discloses a light-emitting device comprising a luminescent region comprising an anthracene derivative compound and a triazine derivative compound.
Phosphorescent materials are also useful and in some applications may be preferable to fluorescent materials. One type of phosphorescent material comprises a host and a phosphorescent emitter in the host. The emitter may be bonded to the host or provided as a separate component in a blend.
Numerous hosts for phosphorescent emitters are described in the prior art including “small molecule” hosts such as 4,4′-bis(carbazol-9-yl)biphenyl), known as CBP, and (4,4′,4″-tris(carbazol-9-yl)triphenylamine), known as TCTA, disclosed in Ikai et al. (Appl. Phys. Lett., 79 no. 2, 2001, 156); and triarylamines such as tris-4-(N-3-methylphenyl-N-phenyl)phenylamine, known as MTDATA. Homopolymers are also known as hosts, in particular poly(vinyl carbazole) disclosed in, for example, Appl. Phys. Lett. 2000, 77(15), 2280; polyfluorenes in Synth. Met. 2001, 116, 379, Phys. Rev. B 2001, 63, 235206 and Appl. Phys. Lett. 2003, 82(7), 1006; poly[4-(N-4-vinylbenzyloxyethyl, N-methylamino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv. Mater. 1999, 11(4), 285; and poly(para-phenylenes) in J. Mater. Chem. 2003, 13, 50-55.
A problem with known host-phosphor systems is that the host may quench emission from the phosphor. In general, the lower the triplet energy level of the host (relative to the phosphor) then the more likely quenching will occur. Polymerisation can exacerbate this problem by reducing the triplet energy level to below that of a monomer when forming a host polymer. Accordingly, there is a need to produce materials with a high triplet energy level for use as hosts in phosphorescent systems.
JP 2005071983 discloses tris-carbazolyl substituted triazines as a host for Irppy, for example:

In Chemistry of Materials (2004), 16(7), 1285-1291 a tris-carbazolyl compound and other arylamino-substituted triazines are disclosed as hosts for Irppy in OLEDs, for example:

WO 2005029923 discloses tris-benzimidazolyl substituted triazine as a host for the red phosphor, Ir (piq):

US2005/0287393 discloses a light emissive composition comprising a phosphorescent dopant and a host including a carbazole compound and a small molecule triazine compound. Exemplary triazine compounds are given as 2,4,6-tris(diarylamino)-1,3,5-triazine, 2,4,6-tris(diphenylamino)-1,3,5-triazine, 2,4,6-tricarbazolo-1,3,5-triazine, 2,4,6-tris(N-phenyl-2-naphthylamino)-1,3,5-triazine, 2,4,6-tris(N-pheyl-1-naphthyl amino)-1,3,5-triazine, or 2,4,6-trisbiphenyl-1,3,5-triazine.
All of the above-identified small molecule triazines for use as hosts in phosphorescent compositions are amino derivative except for the last example, 2,4,6-trisbiphenyl-1,3,5-triazine, given in the list of examples in US2005/0287393. The amino derivatives all have a nitrogen atom directly bonding the aryl substituents to the central triazine ring. However, the present applicant proposes that this nitrogen linkage is not completely stable which can reduce the lifetime of these compositions. Furthermore, none of the above-identified small molecule compounds are soluble enough to be readily solution processed and instead these compounds are more suited to vacuum thermal deposition.
Host-emitter systems are not limited to phosphorescent devices. A wide range of fluorescent low molecular weight metal complexes are known and have been demonstrated in organic light emitting devices [see, e.g., Macromol. Sym. 125 (1997) 1-48, U.S. Pat. Nos. 5,150,006, 6,083,634 and 5,432,014].
As with phosphorescent systems, a problem with known host-fluorescent emitter systems is that the host may quench emission from the fluorescent emitter. It is advantageous to provide a host having a higher LUMO than that of the emitter to inject electrons into the emitter. It is advantageous to provide a host having a lower HOMO than that of the emitter to inject holes into the emitter. Accordingly, there is a need to produce materials with a large band gap between the HOMO and LUMO for use as hosts in fluorescent systems.
Another factor affecting the performance of opto-electronic devices is morphology of the films which make up the device. For semiconductive organic materials it is advantageous to have an amorphous rather than a crystalline film. However, it is desirable not to have too much disorder in the film in order to achieve a device with better performance. Accordingly, there is a desire to produce materials with better film forming characteristics.
It is an aim of the present invention to solve one or more of the problems outlined above.